Lithium carbonate is widely and variously used as, for example, a compounding ingredient of a heat-resistant glass or an optical glass, a ceramic material, a raw material of a lithium secondary battery used in a battery of a mobile phone or a laptop personal computer, a material of an electrolyte, and a raw material of lithium niobate single crystal or lithium tantalite single crystal used in semiconductive laser. For those uses, properties (characteristics) required in the lithium carbonate are of great variety, and vary depending on the purpose of use.
For example, when the lithium carbonate is used as the electronic material or optical industrial material, if the lithium carbonate contains a large amount of impurities, electric properties and optical properties are deteriorated. For this reason, high purity lithium carbonate containing less impurities is required. Lithium carbonate having a purity of 97% or more, preferably 98% or more, more preferably 99% or more and still more preferably 99.5% or more is required as the material of a lithium secondary battery.
Furthermore, high purity lithium carbonate having the content of different metals and other impurities in a level of several ppm, particularly 1 ppm or less, may be required depending on the purpose of use.
The lithium carbonate is produced from lithium resources that naturally exist, and examples of resources in which lithium exists in a large amount with high concentration include lithium deposits and brines in intercontinental salt lakes. In the present circumstances, lithium is mainly produced using brines in intercontinental salt lakes (see Non-Patent Documents 1 and 2).
In connection with the production of lithium carbonate from the brines, a lithium battery is in the spotlight as a drive power source with the progress of the development of electric vehicles, and brine as a lithium source is again noticed as a supply source in mass consumption of lithium carbonate (see Non-Patent Documents 1 and 2).
Salt lakes from which the brine is obtained are located on a limited area such as People's Republic of China, United States of America, Chile, Argentina and Bolivia, and are unevenly distributed.
Particularly, lithium deposits in the salt lakes in the area of the Andes, such as Chile (Salar de Atacama), Argentina (Salar del Hombre Muerto) and Bolivia (Salar de Uyuni) are far better (see Non-Patent Document 1). In fact, a large amount of lithium carbonate is produced using the brines in these areas as a lithium raw material (see Non-Patent Document 2).
The lithium (Li) concentration in the brines of salt lakes in the Andes is from about 0.05 to 0.3 wt %, and after concentrating the brines to a concentration of about 6 wt % by the sunlight, the concentrated brines are used in the production of the lithium carbonate. In such a case, lithium in the form of lithium chloride is used in the production of the lithium carbonate.
The brines contain sodium, potassium, magnesium and the like in high concentration, other than lithium. It is necessary to separate and remove those components in order to produce high purity lithium carbonate, and even in the conventional technologies, those components are separated before or after a carbonation reaction.
The lithium carbonate is required to have high purity as described above, and lithium carbonate having an impurity content of 1 ppm or less is sometimes required as described above.
Examples of a method for producing such high purity lithium carbonate include a method of conducting microfiltration of an aqueous solution containing lithium bicarbonate obtained by reacting crude lithium carbonate with carbon dioxide, and then heat-treating the aqueous solution containing the lithium bicarbonate to deposit lithium carbonate (see Patent Document 1) and a method of treating an aqueous solution containing lithium bicarbonate obtained by reacting crude lithium carbonate with carbon dioxide, with an ion-exchange module, and then heat-treating the aqueous solution containing the lithium bicarbonate to deposit lithium carbonate (see Patent Document 2).
In producing the lithium carbonate, lithium chloride in brine as a lithium resource and sodium carbonate as a raw material of a carbonation reaction are generally used. For this reason, sodium carbonate corresponding to the amount of lithium chloride used that is a raw material of lithium is required in the production.
As a result, in order to produce the lithium carbonate, it is necessary to transport sodium carbonate to the Andes highland exceeding 3,000 meters above sea level, that is the actual place at which lithium chloride brine is produced, or to transport concentrated brine to the place at which a reaction raw material such as sodium carbonate is easily available. In any case, the transportation cost of those greatly affects the production cost of lithium carbonate. When the concentrated brine is transported as in the latter, transportation amount is far increased as compared with the case of transporting sodium carbonate, and the cost is further increased.
As a result of earnest investigations on the above-described problems, the present inventors succeeded the development of a method for producing high purity lithium carbonate that reduced production costs, thereby solving the problems, already filed a patent application, and obtained a patent (see Patent Document 3).
In the production method, it was succeeded to reduce transportation costs by using local resources as much as possible and reusing substances by-produced in a carbonation step, without transporting sodium carbonate to the periphery of salt lakes in the Andes. Specifically, the method is to conduct a carbonation reaction of lithium-containing brine using carbon dioxide gas and ammonia, and it was succeeded to supply both raw materials at the actual place at which lithium chloride brine is produced, by using limestone as a raw material of carbon dioxide gas and a by-product as a raw material of ammonia.
The production method is specifically described below. Namely, the production method includes: mixing ammonia and carbon dioxide gas (carbonate gas) with an aqueous solution containing lithium chloride to conduct a carbonation reaction; and recovering a solid formed after the reaction through solid-liquid separation, in which, as the carbon dioxide gas, a product obtained by thermally decomposing limestone at the actual place at which a carbonation reaction is conducted is used, and, as the ammonia, a product produced by reacting ammonium chloride by-produced in the production of lithium carbonate with quicklime by-produced in the production of carbon dioxide gas or slaked lime obtained by hydrating the quicklime, is used.    Patent Document 1: JP-A-62-252315    Patent Document 2: JP-T-2002-505248    Patent Document 3: Japanese Patent No. 5406822 (JP-A-2012-116681)    Patent Document 4: Japanese Patent No. 5406955 (JP-A-2013-193940)    Non-Patent Document 1: GSJ Chishitsu News No. 670, pages 22 to 26, “Lithium Resources”    Non-Patent Document 2: GSJ Chishitu News No. 670, pages 49 to 52, “Production of Lithium from Salar de Atacama, Chile, and Use of Lithium Compounds”